A process of developing an electrostatic image in electrophotography is to form an image by adhering charged toner particles on the electrostatic image using the electrostatic interaction of the electrostatic image. The developer for developing the electrostatic image includes a one-component developer using a magnetic toner comprising a magnetic material dispersed in a resin and a two-components developer which is used by mixing a non-magnetic toner with a magnetic carrier. The latter is preferably used especially for a full-color copying machine for which high image quality is demanded or a full-color image forming apparatus such as a full-color printer and the like.
As the magnetic carrier used in the two-components developer, there is used a coated carrier in which the surface of ferrite particles or a magnetic material-dispersion type resin core is coated with a resin for the purpose of stabilizing the charging amount and improving the endurance of the carrier.
A lot of proposals have been made for the coated carrier, for example, as a durable carrier which prevents charge injection, there is proposed a carrier coated with a fluorine resin obtained by using a specific monomer (Japanese Patent Application Laid-Open No. H10-307430). In this case, uniform coating properties are also improved by using the specific fluorine resin. However, since fluorine has a strong negative charging property and the rising of the charging amount may be slow for a negative toner, especially when an image having a low printing ratio is continuously printed under a low humidity, the charging amount may be increased.
In addition, there is proposed a carrier which is coated with a copolymer of a specific monomer and a methyl methacrylate monomer and has a contact angle of 95° or more for water (Japanese Patent Application Laid-Open No. 2007-279588). Since the charging stability can be achieved and the releasing property can be improved by the use of such a coating resin, the carrier is excellent in durable stability. However, the adhesion between the core and the coating resin is unstable depending on the kinds of the core material and the coating resin is peeled off in some cases. When a machine is allowed to stand for a few days after long-term use especially under high temperature and high humidity environments, the charging amount may be decreased and fog may occur when an image is output.